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Old 04-03-2009, 12:09 PM   #16
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Resi load centers


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Originally Posted by JohnJ0906 View Post
I've only seen one house with a 3 phase service - 1600 amps.

30,000 sq ft "single family" with a pool and outbuildings.
i've seen quite a few homes with 3 phase service, some delta, some wye....i just don't understand why they didn't use a trough and then feed a single phase panel, that was the strange thin...this house was about 2000 sq ft, and the old a/c comp. was 3 phase delta

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Old 04-03-2009, 01:31 PM   #17
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Resi load centers


How would I find out the available short circuit current
or the minimum required interrupting capacity of the main resi. breaker for this service?

It seems to be between 22,000A and 200,000A. . .?

I'm trying to work backwards to this
http://en.wikipedia.org/wiki/Thévenin's_theorem
equivalent impedance.
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Old 04-03-2009, 02:11 PM   #18
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Resi load centers


Quote:
Originally Posted by Yoyizit View Post
How would I find out the available short circuit current
or the minimum required interrupting capacity of the main resi. breaker for this service?

It seems to be between 22,000A and 200,000A. . .?

I'm trying to work backwards to this
http://en.wikipedia.org/wiki/Thévenin's_theorem
equivalent impedance.
The interrupting capacity of a service is determined by the available fault current. Fault current being determined by the internal impedance of the transformer, the resistance of the connecting conductors, and the service voltage. And whether the short is an arcing fault or a bolted fault.

The power company engineer is the person to ask about any particular service's available fault current
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Old 04-03-2009, 03:16 PM   #19
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Resi load centers


Quote:
Originally Posted by InPhase277 View Post
The interrupting capacity of a service is determined by the available fault current. Fault current being determined by the internal impedance of the transformer, the resistance of the connecting conductors, and the service voltage. And whether the short is an arcing fault or a bolted fault.

The power company engineer is the person to ask about any particular service's available fault current
Yeah, it's sure looking that way.
For either bolted or arcing fault, for ball park, 10KA to over 65KA interrupting rating seems common, with
http://books.google.com/books?id=CIm...num=7#PPT69,M1
100kA for 100A or 200A service. Ray C. Mullin even talks about 240 KA.

240v/100kA = 2.4 milliohm source impedance gives a 24 mV change with a 10A load. Worst case, 240 mV drop at the panel with a 10A load with the 10KA rating.

I think this is close enough for a pass/fail spec for resi. system impedance. If there is a bad connection these values will probably be way higher, so the good readings are hopefully clumped and the bad readings are scattered but all of them way higher.

Using an elec. dryer or elec. water heater as a test load will be even more accurate.
One of these days I should check my own house (which has no symptoms). Real world numbers beat theory any day.

Last edited by Yoyizit; 04-03-2009 at 03:31 PM.
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Old 04-03-2009, 11:34 PM   #20
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Resi load centers


A single phase transformer has to be huge to have more than 10,000 amps of fault current.

A typical pole mount transformer will have about 7% impedance, a pad mount will be slightly less.

A 100KVA transformer operating at 240 volts will have a full load current of 416 amps. If the impedance is 5%, the short circuit current will be 8320 amps. This assumes an infinite source (full voltage at the primary), and no losses in the secondary conductors, or across the main breaker.

The POCO around here will do a fault current study free of charge. They will tell you how much short circuit current is available at their meter. In my experience, a 120/208 or 120/240 service has to be pretty good sized to come up with 10,000 amps.

Rob
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Old 04-04-2009, 10:15 AM   #21
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Resi load centers


So maybe the CB interrupting ratings have a 2x or more safety factor.
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Old 04-04-2009, 04:29 PM   #22
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They might, though I've seen breakers destroyed by interrupting less than their rated current. Well, the available fault current was calculated to be less.....

As sort of a side note, most manufacturers recommend replacing any breaker that has interrupted a fault-level current. Considering the intensity of the arc and the loud bang it makes, this is likely a good idea!

I've tested maybe a dozen or so breakers of all sizes that have interrupted fault-level current, not one has passed the fall-of-potential test. The contacts have been damaged beyond repair. Even if the breaker is cycled a bunch of times, there's still too much voltage drop across the contacts.

Rob

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